Advances in photography in the past two decades are attributable, in large part, to advances made in the technology of hardeners. These advances in hardener technology have made possible, in photographic processing, the use of higher solution temperatures and the combination or elimination of one or more steps.
Photographic materials typically have several photographic layers, such as silver halide emulsion layers, protective layers, filter layers, intermediate layers, and undercoating layers, layered or coated successively on an underlying layer of support material, such as a paper; cellulose ester, acetate or acetobutyrate; polyester; polycarbonate; glass; metal; or the like. The photographic layers are typically aqueous solutions of polymers, such as gelatin, in which other components, such as dyes, color couplers, sensitizers, silver halide crystals, or the like, as well understood in the photographic arts, are dissolved or dispersed. Among other functions, the polymer in the layers provides a modicum of structural integrity to the layers and the photographic material consisting of the layers.
During processing, after exposure, photographic materials are typically passed through several aqueous solutions, all of which may have different pHs and possibly elevated temperatures. In the course of this processing, the layers of a photographic material must not dissolve, swell excessively, delaminate or separate from the support. If any of these result, the performance of the photographic material is severely compromised. The layers of a photographic material must thus be made resistant to such degradative processes Resistance to such processes is achieved in part by "hardening" the layers.
"Hardening" a layer of a photographic material means treating the layer so that it acquires a higher "melting point", i.e a higher temperature required for dissolution of the layer in water "Hardening" entails crosslinking molecules of the polymers, such as gelatin, which serve as "binders" or "colloidal carriers," in the photographic layers.
A variety of substances are capable of reacting with the protein molecules of gelatin to effect crosslinking and, thereby, hardening of layers in photographic materials. Such hardeners include salts of certain metals, such as zirconium and chromium salts; bifunctional aldehydes, ketones, sulfonate esters, sulfonyl halides, carboxylic acid derivatives, and carbonic acid derivatives; carbodiimides; and isoxazolium salts See, e.g. James, The Theory of the Photographic Process, 4th ed. (1977), pp. 77-87. The use of these so-called "conventional" hardeners, however, involves a number of problems.
One problem common to most conventional hardeners is their relatively slow rate of reaction with the gelatin, resulting in hardening over a prolonged period of time. Such slow reaction velocity results in afterhardening, ie, increased hardening upon storage of the photographic materials, causing sensitometric changes such as reduction in contrast To attempt to solve this problem, so-called "fast-acting hardeners" (also known as rapid-acting hardeners or quick-acting hardeners) have been described and used.
These fast-acting hardeners have a high reaction rate in crosslinking gelatin and cause hardening within a relatively short time after their addition to gelatin solutions. With fast-acting hardeners, it is possible to avoid afterhardening during storage of photographic materials.
Many fast-acting hardeners have been described. For example, U.S. Pat. No. 4,119,464 describes certain carbamoylonium compounds, carbamoylpyridinium and carbomoyloxypyridinium salts. U.S. Pat. No. 4,067,741 discloses the use of certain sulfonyl pyridinium compounds, while U.S. Pat. No. 4,063,952 discloses the use of sulfo- or sulfoalkyl-substituted carbamoyl pyridinium compounds. European Patent Application Publication No. 257,515 discloses hardeners which are carbamoyl and ureido compounds.
However, fast-acting hardeners react so quickly with gelatin that they pose several other problems in the production of photographic materials. For example, the use of fast-acting hardeners causes difficulties in the coating of the photographic layers in such production. When fast-acting hardeners are added to a solution to be coated, their reaction with gelatin may be so rapid that hardening takes place in the coating hopper apparatus, producing slugs of hardened gelatin and resulting in nonuniform coatings.
Thus, it would be desirable to provide a delivery system for fast-acting hardeners which would allow rapid hardening of coated gelatin-containing layers as compared to conventional hardeners, yet avoid the onset of premature hardening within the coating hopper.
Approaches have been described to avoid the problems arising from the short reaction time of fast-acting hardeners. For example, U.S. Pat. No. 4,233,398 discloses the incorporation of a polysaccharide with a fast-acting hardener which can then be coated on top of a gelatin layer to be hardened. U.K. Patent No. 1,275,587 discloses the addition of copolymers of acrylic acid and an alkyl acrylate in the layers containing the fast-acting hardener. However, U.S. Pat. No. 4,233,298 discloses that use of such compounds increases swelling in the layers, especially when used with carbodiimide and isoxazolium hardeners.
Further problems for the use of fast-acting hardeners are presented by the need for compatibility, of whatever chemical or physical measures are taken to control excessively rapid hardening, with methods used to coat layers in the course of making a photographic material and with preservation of the sensitometric quality of a photographic material.